1. Field of the Invention
The present invention relates generally to automatic gain control circuits, and more particularly, to an automatic gain control circuit exhibiting a decrease in the intermodulation distortion level and an increase in the signal-to-noise ratio under automatic gain control.
2. Description of the Prior Art
The conventional automatic gain control circuit operates to maintain constant the level of the received signals, whereby the average level of the output audio signal from the receiver is maintained constant. In other words, the automatic gain control circuit results in the signal level of the audio output signal being substantially constant and, thus, immune to fluctuations in the level of the received signal. The constant level of the audio signal is a very attractive feature, and this accounts for the fact that almost all currently available radio receivers include automatic gain control circuits.
One type of conventional automatic gain control circuit is so designed that the gate or source of an active device, such as a field effect transistor, used in a preamplifier stage for amplifying the radio frequency signals present at the antenna input terminal, is provided with an automatic gain control signal to maintain substantially constant the level of the output signal from the preamplifier stage.
The automatic gain control signal so provided has a direct current component and also has an alternating current component when the level of the radio frequency signals present at the antenna input terminal or the like is rapidly fluctuating. Thus, in the conventional automatic gain control circuit described above, the gate and the source of the field effect transistor usually have different direct current potential levels. The different potential levels result in the conventional automatic gain control circuit exhibiting an increase in the intermodulation distortion level and a decrease in the signal-to-noise ratio under automatic gain control.